In recent years, use of a “pre-applied underfill film (PUF)” that is an underfill film applied to electrodes of a semiconductor integrated circuit (IC) has been examined for the purpose of shortening processes in semiconductor chip mounting methods.
A mounting method using a pre-applied underfill film is carried out as follows, for example (see PLT 1, for example).
Step A: An underfill film is applied to a wafer, and the resulting wafer is diced into semiconductor chips.
Step B: A semiconductor chip with the underfill film applied thereto is positioned and mounted.
Step C: Thermocompression bonding is conducted on the semiconductor chip, so that electrical conduction is ensured through metallic bond of solder bumps and the semiconductor chip is bonded as a result of curing of the underfill film.
The viscosity of the underfill material lowers (liquefied) with temperature rise until reaction starts, and increases from a reaction starting point, and the underfill material turns into a hardened material. As a result of such changes in the viscosity, voids easily disappear; however, voids are likely to remain when the pressure is changed at wrong timings. In order to apply pressure in a timely manner, adjustment is typically made according to a mounting profile. Since an optimum condition is determined by the timing of pressure application and the viscosity of the underfill material, an optimum mounting condition needs to be found through actual mounting using chips.
For examination of the above, data from a rheometer are typically used. For example, among melt viscosity curves shown in FIG. 10, NCF1 is suitable for mounting at low temperature in a short time, while NCF3 is suitable for mounting at a high temperature in a long time. Since, however, the temperature increase rate of the rheometer is greatly different from that in mounting, it is difficult to determine whether or not the curves are suitable for voidless mounting only from the rheometer data.
Since voidless mounting is affected by the shape and the heat conduction, among others, of actual packages, underfill materials that enable voidless mounting are likely to vary from one another. Furthermore, typically, an underfill material can achieve good mounting only with a predetermined mounting profile, which makes the margin for mounting narrow.